Titanite, also known as Sphene (especially when cut for gemstones), is a rare mineral occasionally mined as a source of titanium dioxide, used to create white pigments. Exceptional, gem-quality crystals, which are usually somewhere on the green-yellow spectrum, are highly sought after by collectors. Cut titanite has extremely high dispersion, or "fire" (i.e., white light passing through it is dispersed into many colors - diamonds are also famous for having high dispersion).
